"I will kill people in Akihabara," wrote factory worker Tomohiro Kato, 25, in a message posted via cell phone to an online bulletin board on June 8. Then he apparently did just that. Dressed in a pale suit, he drove about 60 miles (95 km) from his home to the popular shopping district and plowed his rented truck into a crowd before leaping out of the vehicle and frantically stabbing innocent bystanders. Seven people were killed in the incident, which followed a disquieting series of random stabbings in recent months in a nation where violent...